X-ray imagers using thin film transistors (TFTs) have been developed for X-ray imaging systems, which are widely used in various fields of life and industry. FIG. 1 illustrates a conventional X-ray imager 10, which comprises a plurality of amorphous or polycrystalline TFTs 12 and capacitors 14 formed on a substrate 16. A pixellated electrode 18 is deposited atop the TFT and capacitor array. Each TFT 12 and capacitor 14 is coupled to one pixel 18. The TFTs 12 have gate lines 20 to address the matrix lines and data lines to read out the charge accumulated at each capacitor 14. Directly deposited atop the pixellated electrode 18 is a photoconductor 22, on which a conductive electrode 24 is deposited.
Photoconductor 22 converts X-ray photons into electrical signals. The electron-hole pairs produced by photoconductor 22 are in proportion to the strength of external X-rays. Depending on the voltage polarity applied to conductive electrodes 18 and 24, either electrons or holes are gathered by the pixellated electrode 18 as electric charge. The gathered electric charge is accumulated and stored in capacitors 14. The charge in capacitors 14 is then selectively transferred through TFTs 12 to an external image display device that forms an X-ray image.
Since photoconductor 22 is traditionally directly deposited onto the pixellated electrode 18 and TFT 12 and capacitor 14 array, the temperature tolerance of the TFTs and capacitors limits the maximum temperature of deposition of the photoconductive material 22. The most widely used amorphous silicon (a-Si) TFT array for imaging can withstand only 200 to 250° C. However, some good photoconductor materials such as Cadmium Zinc Telluride (CZT) require a much higher deposition temperature (e.g., more than 600° C.) to achieve good quality polycrystalline photosensitive layer.
It has been proposed that a photoconductive material is deposited on a separate substrate, which is then bonded onto a TFT array. FIG. 2 schematically shows a prior art method in which metal indium (In) balls 26 are used in the bonding. As shown in FIG. 2, an indium ball 26 is applied onto each of a plurality of pixels 18. The TFT 12 and capacitor 14 array is then heated to melt the indium balls 26. The substrate 25 deposited with a photoconductive layer 22 is then pressed against the TFT 12 and capacitor 14 array to provide bonding between the photoconductor layer 22 and the pixellated electrode 18. One limitation of this method is that it requires millions of indium balls precisely aligned pixel by pixel on a large area device (e.g., 17″×17″), which is difficult to achieve. If the surface of the photoconductor layer is not sufficiently flat and smooth, for example, to a few microns, or the indium balls are slightly misaligned as shown in FIG. 2 (the middle ball), then either open circuits or shorts between neighboring pixels are resulted after the bonding. Another limitation is that some photoconductive materials cannot withstand the high temperature of the bonding process and/or are incompatible with indium material.
An alternative method is shown in FIG. 3 in which a conductive resin 28 is used to bond a photoconductor layer with a TFT array. One problem of this method is that the conductive resin 28 also conducts sideways or horizontally. Sideway conduction results in signal sharing between adjacent pixels, and thus degrading the spatial resolution and pixel-to-pixel homogeneity.
Accordingly, there is a need for improved X-ray imagers that eliminate these and other problems and/or limitations of prior art X-ray imager.